As the immune cells of the central nervous system (CNS), microglia are constantly surveilling their environment. The traditional thinking has been that they are activated in times of stress, trauma or disease. However, a growing body of evidence has revealed a more prominent role in the developing brain. This study demonstrates the roles of microglia in synaptic remodeling in the postnatal, but still developing brain by becoming active during developmental pruning of synaptic contacts. The implications for circuitry are profound even into adulthood, but there are also possible direct associations with neuron/glia interactions and disorders such as amblyopia that affects vision in children when their eyes are not properly aligned.